Field of the Invention
The invention relates to biometry, the study of characteristics of people for identification purposes. More specifically, the invention relates to active illumination used during biometry, especially iris and facial recognition, for enhancing user experience while maintaining image quality.
Description of the Related Art
Active illumination has been widely used in the field of iris recognition, which is the field of recognizing individuals based on the patterns of the iris in the human eye. For example, Daugman describes a range of iris recognition deployments all using active infrared illumination [J. Daugman/“The Importance of being Random: Statistical Principles of iris Recognition”/Pattern Recognition 26 (2003) 279-291]. A problem however is that the illumination is often noticed by the subject, which may cause them some temporary discomfort while using the system.
Moreover, FIG. 1 shows methods for iris recognition using pulsed lighting 11 synchronized to frame acquisition 10 that has been described in US 2003/0169334 A1 and U.S. Pat. No. 7,542,628, for example, as a means to stop the motion of an individual while performing iris recognition. The top graph on the horizontal axis shows time, and on the vertical axis it is shown whether a frame is being acquired or not. In this example three frames of a continuous sequence are shown being acquired, with each frame being acquired in a finite time period T.
The illumination in these systems is more noticeable to the user due to the repetitive on/off cycle of the illumination. Pulsed Light Emitting Diode (LED) lighting compared to constant LED lighting is preferable in these applications since for a given average heat dissipation capability of an LED, more power can be concentrated in the pulse during which the frame is being acquired, resulting in higher quality imagery with a lower signal to noise ratio, rather than wasting the power during the time period when the image is not being acquired.
In addition, as iris recognition devices have becomes smaller, a side effect is that the user is more likely to look directly at or near the light sources mounted within a compact case. As such, the impact of the illumination is perceptually much greater than if the illumination were placed elsewhere. Put simply, the illuminators are more noticeable to the user, even though the incident power to the eye is the same compared to when the illuminators are placed elsewhere.
Of equal importance, the power of Light Emitting Diode (LED) illumination has increased substantially in recent years so that the LEDs are that much more noticeable to the user even though the incident power is the same, compared to less powerful LEDs spread over a larger area. High power LEDs can now be pulsed at >=250 mA. We have found that the use of pulsed illumination combined with the two factors above vastly increases the user's perception of the illumination. This very high perception of the illumination even given safe illumination intensity level can not only be annoying to a user, it can also create photosensitive epilepsy in certain subjects.
Wilkins in “Visual Stress”, Oxford Univ Press, 1995 describes how the peak response for photosensitive epilepsy is approximately 15 Hz, and the wavelength of light to which patients are most sensitive is in the red wavelength region, which is near the infra-red region used for iris recognition.
For all the aforementioned reasons therefore, it is important to reduce the visibility of illumination to the subject, while not impacting the quality of imagery acquired. This is a difficult problem since changing the characteristics of the illumination can potentially adversely impact the characteristics of the images being acquired.